1. Field of the Invention
The present invention relates to a correction circuit and a display device that correct display non-uniformity occurring in a display device in which display elements are arranged in matrix.
2. Description of the Related Art
In an organic EL display device, organic EL elements, which are self-emissive elements, are used as pixels. A luminance level (gradation) of each of the organic EL elements, that is, light emitting elements, arranged in matrix can be controlled by a current flowing in the elements. Thus the organic EL display device is a current controlled device (current drive method) and therefore greatly differs from a voltage controlled device such as a liquid crystal display device.
Organic EL display devices employ a passive matrix method or an active matrix method as a driving method thereof. In recent years, organic EL display devices employing the active matrix method have been extensively developed. In the active matrix method, a current flowing in a light emitting element in each pixel circuit is controlled by an active element provided in a pixel circuit. A thin film transistor (TFT) is commonly used as the active element, and is called a driving transistor due to its function.
In a TFT panel in which TFTs are arranged in matrix, a relationship between potential of an input signal and light emitting luminance in each pixel corresponds to a relationship between a gate applied voltage and a drain current in a driving transistor of a pixel (see Japanese Unexamined Patent Application Publication No. 2006-84899, for example).
An operating characteristic of a driving transistor is expressed as Expression 1 below.Ids=(½)μ(W/L)Cox(Vgs−Vth)2  (1)
In Expression 1, Ids denotes a drain current flowing between a source and a drain, that is, an output current supplied to a light emitting element in a pixel circuit. Vgs denotes a gate voltage applied to a gate with reference to the source, that is, input potential mentioned above in the pixel circuit. Vth denotes a threshold voltage of the transistor. μ denotes mobility of a carrier in a semiconductor thin film constituting a channel of the transistor. W denotes a channel width, L denotes a channel length, and Cox denotes a capacitance.
In a TFT including a semiconductor thin film of polysilicon, a threshold voltage Vth and mobility μ (V-I) characteristic) commonly have a variation (see Japanese Unexamined Patent Application Publicastion Nos. 2006-84899 and 2007-18876, for example). Tha variation of the threshold voltage Vth and the mobility μ cause luminance non-uniformity for every pixel, causing color non-uniformity and display non-uniformity.
In recent years, a silicon film of a polysilicon TFT is commonly formed by a laser annealing method in which amorphous silicon is crystallized by laser. However, a crystalline semiconductor film formed by the method has a structure including a plurality of crystalline grains. It has been difficult to control positions and sizes of the crystalline grains (see Japanese Unexamined Patent Application Publication No. 2008-252101, for example). The distribution characteristic of the crystalline grains influences both of mobility of a carrier in a channel region and a threshold voltage of the transistor (see Japanese Unexamined Patent Application Publication No. 2008-252101 and “Statistical Analyses of the Influence of Grain Boundary Variations in Poly-Si TFTs”, Technical Report of IEICE, VLD, VLSI Design Technologies, The Institute of Electronics, Information and Communication Engineers, Vol. 102 (No. 344), pp. 25-30 (Sep. 23, 2002), for example).
In a case of correction of luminance non-uniformity by signal processing, the correction has been commonly performed by calculating these two values (see Japanese Unexamined Patent Application Publication Nos. 2006-84899, 2004-264793, and 2007-18876). FIG. 1 illustrates characteristic curves showing a relation between an input signal voltage and light emitting luminance in a case where a threshold voltage in one pixel is shifted from a threshold voltage of the other pixel by Vth′ and mobility in the one pixel is multiplied by μ′ with respect to mobility in the other pixel, in two pixels. In FIG. 1, a horizontal axis indicates an input signal voltage V and a vertical axis indicates an output current I (corresponding to output luminance). In FIG. 1, a characteristic curve 2a, which is drawn by a dashed line, of a specific pixel is an example of a curve in a case where a threshold voltage is shifted by Vth′ with respect to a characteristic curve 1 of an adjacent pixel (a part of an arrow in a horizontal direction). A characteristic curve 2 is an example of a curve in a case where the output current I is corrected so that the mobility is multiplied by μ′ with respect to the characteristic curve 2a (a part of an arrow in an upward direction).
In this case, the output current I corresponding to a part of the characteristic curve 2 in an area where the light emitting luminance of an intended pixel changes linearly with respect to the input signal voltage (neighborhood of a dashed-dotted line) is multiplied by Δμ, which satisfies an equation Δμ=1/μ′. Then, ΔVth, which satisfies an equation ΔVth=−Vth′ is added to the input signal voltage V corresponding to a part of the characteristic curve 1. By doing this operation on the basis of Expression 1, accurate correction may be achieved.
FIG. 2 illustrates a block diagram for correcting a threshold voltage and mobility.
A correction circuit 20 shown in FIG. 2 corrects luminance data on the basis of mobility correction data that is pre-stored in a memory 22a and threshold voltage correction data that is pre-stored in a memory 25a so as to supply the corrected luminance data to a display panel 10 (TFT panel).
The display panel 10 has a pixel of respective colors of red, green, and blue (RGB). Input data (pixel data: luminance data) which are voltage signals of luminance of each pixel are inputted separately for each of the colors of RGB, whereby the display panel 10 is capable of controlling a display of every color. Here, a coordinate of a dot in a display area is denoted as (X, Y).
R data, G data, and B data are respectively supplied to a multiplier 21R, a multiplier 21G, and a multiplier 21B. To the multipliers 21R, 21G, and 21B, correction values Δμ for correcting variation of mobility for every pixel are respectively supplied. The correction values are read out from the memory 22a by a memory read-out unit 22 on the basis of a coordinate signal (X coordinate, Y coordinate).
Outputs of the multipliers 21R, 21G, and 21B are supplied to square root operation units 23R, 23G, and 23B determining a square root. Outputs of the square root operation units 23R, 23G, and 23B are respectively supplied to adders 24R, 24G, and 24B.
To the adders 24R, 24G, and 24B, correction values ΔVth for correcting variation of threshold voltages for every pixel are respectively supplied from a memory read-out unit 25 which reads out the correction values ΔVth from the memory 25a on the basis of the coordinate signal (X coordinate, Y coordinate).
Then outputs of the adders 24R, 24G, and 24B are respectively supplied to D/A converters 26R, 26G, and 26B and converted into analog data signals so as to be supplied to input terminals of respective colors in the display panel 10. Consequently, an organic EL element is driven in each pixel by currents corresponding to the data signals of respective colors that are corrected for every pixel.
As above, luminance non-uniformity occurring in an organic EL element due to a problem in manufacturing may be corrected. However, as mentioned above, two correction values of the mobility μ and the threshold voltage Vth are stored in a memory for every pixel, resulting in a problem of greatly large-size data depending on the number of pixels.
In view of the above, Japanese Unexamined Patent Application Publication No. 2004-264793 discloses a display device in which a display area is divided into small areas in a display panel having a large number of pixels. In the device, a coefficient for correcting the whole of the display area is calculated by measuring a current in each of the small areas and estimating a trend of the whole of the display area, or correction is performed in each of the small areas.